1. Field of the Invention
The present invention relates to an electrically programmable nonvolatile storage device, and more particularly, to a nonvolatile magnetic memory device.
2. Description of a Related Art
In a nonvolatile magnetic memory device of this kind, each memory cell has a magnetic tunneling junction (MTJ) formed by interposing a thin insulating layer between a pair of ferromagnetic layers. When a voltage is applied between the two ferromagnetic layers, electrons in one of the ferromagnetic layers enters the other by passing through the insulating layer by quantum mechanical tunneling. Thus, a xe2x80x9ctunneling currentxe2x80x9d flows through the magnetic tunneling junction.
The electrical resistance of the magnetic tunneling junction changes according to the directions of magnetization of the two ferromagnetic layers. That is, the electrical resistance of the magnetic tunneling junction has a minimum value when the directions of magnetization of the two ferromagnetic layers are in parallel with each other, and has a maximum value when the directions of magnetization of the two ferromagnetic layers are in inverse parallel. Therefore, if the direction of magnetization of one of the ferromagnetic layers is changed by an applied magnetic field (external magnetic field) while the direction of magnetization of the other ferromagnetic layer is fixed, the electrical resistance value of the magnetic tunneling junction changes according to the direction of the applied magnetic field. That is, the value of the tunneling current changes. Thus, information is stored in each memory cell and read out (reproduced) from the cell by sensing the tunneling current value.
The thickness of each of the two ferromagnetic layers is freely selected, but it is necessary to set the thickness of the insulating layer to an extremely small value of about several nanometers since the quantum mechanical tunneling phenomenon is utilized.
For example, Japanese Patent Application Laid-open No. 2000-82791 discloses the principle of operation of Such a nonvolatile magnetic storage device.
Ordinarily, in a nonvolatile magnetic storage device of this kind, a multiplicity of memory cells each using the above-described magnetic tunneling junction are arranged in matrix form and upper and lower wiring layers are laid over and under the memory cells in substantially parallel with each other while being spaced apart at a predetermined distance from each other. The upper wiring layer formed of a low-electrical-resistance conductive material is patterned to form a plurality of bit lines in a predetermined configuration. Similarly, the lower wiring layer formed of a low-electrical-resistance conductive material is patterned to form a plurality of word lines in a predetermined configuration, which intersect the bit lines at right angles. The thus-formed nonvolatile magnetic storage device is called a magnetic random access memory (MRAM).
Each memory cell in the MRAM has two ferromagnetic layers. One is a storage layer having a direction of magnetization changed according to the direction of an external magnetic field. The storage layer is electrically connected to the corresponding bit line. The other layer is a pinned layer having a fixed direction of magnetization. The layer is electrically connected to the corresponding word line. When information is recorded (written) in a one of memory cells selected as desired, the word and bit lines electrically connected to the memory cell are selected and predetermined write currents are caused to flow respectively through the word and bit lines. These write currents induce magnetic fields around the word and bit lines according to the values of the write currents. The direction of magnetization of the upper ferromagnetic layer, i.e., the storage layer, changes according to a resultant magnetic field formed by the two induced magnetic fields.
If the direction of magnetization changed in this manner is the same as the direction of magnetization of the lower ferromagnetic layer, i.e., the free layer, of the same memory cell, the directions of the free layer and the storage layer are in parallel. If the changed direction of magnetization is opposite to the direction of magnetization of the free layer, the directions of the free layer and the storage layer are in inverse parallel. The direction of magnetization of the storage layer is thus changed to write binary information xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d in the selected cell.
To change the value written in the selected memory cell, one of the write currents caused to flow through the word and bit lines is reversed in direction (inverted). The direction of resultant magnetic field induced around the word and bit lines by the two write currents is thereby changed to reverse the direction of magnetization of the storage layer, i.e., to write the other value.
Ordinarily, in MRAMS, the upper wiring layer forming bit lines and the lower wiring layer forming word lines are formed of the same material, e.g., aluminum (Al), copper (Cu), or an alloy having Al or Cu as a major component. The upper and lower wiring layers are formed in this manner by considering the advantages of simplying the manufacture processing, limting the manufacturing cost, etc. However, the inventor of the present invention has found in conventional MRAMS problems described below.
Since the storage cells, the upper wiring (bit lines) and the lower wiring (word lines) are formed and placed as described above, the upper ferromagnetic layer (storage layer) on each memory cell is close to the upper wiring (bit line) and at some distance from the lower wiring (word line).
Generally, the intensity H of a magnetic field induced by a current at a distanced from a stright line along which the current flows in a space is proportional to the magnitude I of the current and inversely proportional to the distance d. That is, H=kI/d (k: a constant). Therefore the intensity of a magnetic field induced in the upper ferromagnetic layer (storage layer) in one memory cell by the current flowing through the corresponding bit line is higher than that of a magnetic field induced by the current flowing through the corresponding word line. To equalize the intensities of the two induced magnetic fields, it is necessary to increase the current flowing through the word line relative to the current flowing through the bit line.
However, sicne the bit line and the word line are made by the same material, the long-term reliability of the word line having the larger current caused to flow therethrough in comparison with the bit line is reduced relative to that of the bit line.
It is an object of the present invention to provide a nonvolatile magnetic memory device in which the intensities of magnetic fields induced in a storage layer in a memory cell by currents flowing through two wiring conductors on the opposite sides of the memory cell can be made approximately equal to each other without any problem in terms of long-term reliability or layout.
It is another object of the present invention to provide a nonvolatile magnetic memory device in which the intensities of magnetic fields induced in a storage layer in a memory cell by currents flowing through two wiring conductors on the opposite sides of the memory cell can be made approximately equal to each other while maintaining the two wiring conductors in the state of being approximately equal to each other in sectional area.
It is still another object of the present invention to provide a nonvolatile magnetic storage device capable of using a design for an optimized memory cell.
A magnetic memory device of the present invention includes a first wiring conductor having a first ability to flow a current therethrough, a second wiring conductor having a second ability larger than the first ability to flow a current therethrough, a magnetic memory cell having a pinned magnetic layer coupled to the second wiring conductor, a free magnetic layer coupled to the first wiring conductor and a non-magnetic layer sandwiched between the first and second magnetic layers.
Japanese Patent Application Laid-open No. 2000-195250 discloses the same kind of nonvolatile magnetic storage device as the first nonvolatile magnetic storage device of the present invention. According to this publication, however, each of the bit lines and word lines in the nonvolatile magnetic storage device is formed of CuAu and only use of a low-resistance wiring material such as Cu or Al as an alternative material for forming the bit lines is mentioned.